Typically, fetal signals that are available for measurement include direct or vaginal fetal ECG signals, indirect or abdominal fetal ECG signals, fetal phonocardiographic signals, and ultrasound doppler fetal heart beat signals. Of these fetal signals, ultrasound doppler fetal heart beat signals seem best suited for clinical applications because they can be obtained non-invasively, without harm to mother or fetus, any time from about 10 weeks of pregnancy until delivery, and further because these signals have an acceptable signal to noise ratio and are not adversely affected by obesity of the mother, movement of the mother, fetal position, or ambient and acoustic noises.
Heretofore, the method of measuring fetal heart rate from an ultrasound doppler fetal heart beat signal required the recognition of a suitable component of the signal, the generation of a trigger pulse having a predetermined temporal relationship to said component, the measuring of the period between trigger pulses, and the calculation of the reciprocal value of said period.
Because an ultrasound doppler fetal heart beat signal includes blood flow, muscle, and valve signals as components of the heart beat signal, and because the envelope of the valve signal component is typically suited for triggering, this valve signal component has often been used as trigger signals. However, because there is more than one valve signal in an ultrasound doppler fetal heart beat signal, and because it is difficult to get a reliable signal because the level of these signals fade from time to time, the use of valve signals as trigger signals often produces ambiguous and incorrect results.